Chloramphenicol-induced aplastic anemia in calves by oral administration of the drug appears to require the presence of either the nitro group or the dichloroacetamide group. Modification of the drug molecule by replacement of either nitro group by methyl sulfonyl group (as in thiamphenicol) or the dichloroacetamide group by trifluoroacetamide group (as in fluoramphenicol) lowers the incidence but does not prevent the toxic effect. The replacement of both nitro group by methyl sulfonyl group and dichloroacetamide group by trifluoracetamide group (as in trifluorothiamphenicol) appears to eliminate the toxic effect of the drug. BIBLIOGRAPHIC REFERENCE: Reddy, B.G., Pohl, L.R. and Krishna, G.: The requirement of gut flora in nitro benzene-induced methemoglobinemia in rats. Biochem. Pharmacol. 22: 119-1122, 1976.